Most of the conventional methods for the instrumental analysis of odors use the component analysis with gas chromatographs or gas chromatograph mass spectrometers. The component analysis, however, is affected by various problems. For example, the measurement takes a long time and requires considerable skill. Furthermore, a large number of measurement signals is obtained for each sample, so that the analysis and interpretation of the signals are difficult and take a long time. Also, the measurement result has no correlation with the organoleptic evaluation by the olfactory sense of a human being.
To solve such problems, some odor measuring apparatuses use odor sensors responsive to odorous substances. Examples of such apparatuses are disclosed in the Japanese Unexamined Patent Publication Nos. H11-352088 and 2002-22692, and “Development of Odor Discriminating Apparatus,” KITA, Junichi et al., Shimadzu Review, Vol. 59, No. 1–2 (November 2002), Shimadzu Corp., pp. 77–85. These apparatuses are capable of processing the detection signals produced by plural odor sensors and determining the distances between the odors of plural samples (i.e. whether or not these odors belong to the same or similar categories) by the cluster analysis, principal component analysis or other type of multivariate analysis, or by the non-linear analysis using neural networks.
For example, in the principal component analysis, a graph representing a space formed by plural axes corresponding to different principal components is created, and the results of the measurements of a large number of samples are plotted in the space as measurement points. The spatial relation between the measurement points enables the determination of the similarity or difference between the odors. However, in principle, the principal component analysis allows only a relative comparison of the sensor outputs. In this analysis, the selection of the samples to be simultaneously measured may influence the measurement result. For example, two relatively similar odors might be located very far from each other on the graph, or inversely, two odors having little similarity might be located close to each other on the graph. In such cases, the user often misunderstands the measurement result. Therefore, it is necessary to considerably limit the kinds of odors to be simultaneously measured.
According to another analysis method, the Euclidean distance between the measurement result of a reference odor and that of a subject odor is calculated, and the similarity or difference between the two odors is determined from the Euclidean distance. This method provides the absolute difference between the two odors, but it is impossible to determine whether the difference results from the quantitative difference or the qualitative difference between the odors. For example, in the evaluation of the odors of food, drinks, cosmetics, medicines or other products, it is impossible to determine whether the difference is due to a mixture of an unexpected odor in the subject odor or an excessive addition of the perfumes concerned. Therefore, the evaluation or examination result cannot always be satisfactory.
In view of the above-described problems, the present invention intends to provide an odor discriminating apparatus capable of determining the similarities or differences between plural odors with respect to the odor quality and the odor intensity. The present invention also intends to provide an odor discriminating apparatus capable of visually representing the measurement result in such a manner that helps the user to correctly understand the result and prevents the user from making a wrong determination.